The present invention relates in general to a ceramic envelope device for use in a high-pressure discharge lamp (hereinafter referred to as "HID lamp"; "HID" representing High Intensity Discharge) having a pair of closure discs in the form of electrically conducting end caps which close the opposite open ends of a translucent ceramic arc tube to form a gas-tight envelope. More particularly, the invention relates to an arc-discharging electrode and an electric-power lead member which are fixed to one of the opposite sides of each closure disc and to the other side of the same, respectively.
In the art of such HID lamps using a translucent ceramic tube, a pair of electrically conducting discs are known as end caps to close the opposite open ends of the translucent ceramic tube. Examples of such closure end caps are illustrated in U.S. Pat. Nos. 4,155,757 and 4,155,758. Such end caps are formed of an electrically conducting cermet obtained by mixing, for example, particles of tungsten with particles of aluminum oxide, and sintering the mixture. The electrically conducting cermet end caps support a pair of tungsten electrodes at their inner surfaces in the interior of the ceramic envelope, so that the electrodes protrude from the inner surfaces of the end caps toward each other, i.e., longitudinally inwardly in the translucent ceramic tube. Additionally power-supply lead rods or contact rods are connected or fixed to the outer surfaces of the cermet end caps by suitable methods, so that electric power is applied to the pair of opposed tungsten electrodes through the contact rods and through the cermet end caps. Such cermet end caps have been advantageously employed, for example, in high-pressure sodium lamps, because they eliminate the need of using expensive metallic niobium. It is further recognized that such cermet end caps have been used also advantageously for so-called metal halide lamps which employ translucent ceramic tubes charged with a suitable metal halide together with mercury and rare gas, because the cermet exhibits relatively high corrosion resistance to metal halides.
However, such cermet end cap has a tendency to crack due to an excessive shrinkage of its green body in a sintering process in which an electrode and a power-supply lead member are partially embedded in the cermet end cap. The cermet end cap is also apt to crack due to too large a difference in thermal expansion between the material thereof and that of the electrode and lead member. Such cracks in turn cause the translucent ceramic tube to leak and thereby lower the degree of its luminous flux. When these problems become severe, the HID lamp will fail to function.